1. Field of the Invention
The present invention relates to a desensitizing gum for lithographic printing plate, more particularly to a desensitizing gum of emulsion type containing a specific polysaccharide derivative and a gumming-up process using the desensitizing gum.
2. Description of the Prior Art
When a lithographic printing plate is prepared, a desensitizing gum, so-called gumming solution, is applied to the surface of the lithographic printing plate as the final step of the preparation thereof The application of such a desensitizing gum is carried out for the purposes of preventing the deposition of any contaminants on the surface of the lithographic printing plate possibly occurring during storage after the lithographic printing plate is produced, which possibly leads to the background contamination of printed matters, as well as preventing the formation of any defects due to the storage thereof in piles or the collision with foreign substances. The application of such a desensitizing gum is an essential process for making lithographic printing plates in view of preventing the nonimage areas of the lithographic printing plate from lowering the hydrophilic property due to oxidation upon being directly exposed to air and further enhancing the hydrophilicity thereof.
As such a gumming solution, a composition comprising an aqueous solution of gum arabic having a concentration of about 15 to 30% by weight and optionally a surfactant, a pH adjusting agent, a preservative and the like has been used over the past years.
However, the gum arabic is a natural product which is obtained only in limited districts and the yield thereof is greatly influenced by a variety of conditions such as the weather in the districts. Therefore, a steady supply thereof is not expected at all.
For this reason, many attempts have been directed to the use of various water-soluble organic polymers in place of gum arabic as the desensitizing gum.
For instance, Japanese Patent Un-examined Published Application (hereinafter referred to as "J. P. KOKAI") No. 54-97102 discloses dextrin, sterabic, stractan, alginates, plyacrylic acids, hydroxyethyl cellulose, polyvinylpyrrolidone, polyacrylamide, methyl cellulose, hydroxypropyl cellulose, hydroxymethyl cellulose and salts of carboxyalkyl cellulose. In addition, Japanese Patent Publication for Opposition Purpose (hereunder referred to as "J. P. KOKOKU") No. 54-41921 discloses pullulan and derivatives thereof; J. P. KOKAI No. 58-197091 discloses polyvinylpyrrolidone; and J. P. KOKAI No. 56-133193 discloses polyvinyl alcohol. Moreover, J. P. KOKAI No. 5957242 discloses the following compounds as modified starch derivatives:
Roasted starch such as British gum; dextrins modified with enzymes such as enzyme dextrin and Schardinger dextrin; acid decomposed starch represented by solubilized starch; alpha-starch such as modified alpha-starch and unmodified alpha-starch; esterified starch such as phosphated starch, fatty starch, sulfated starch, nitrated starch, starch xanthate and starch carbamate; etherified starch such as carboxyalkyl starch, hydroxyalkyl starch, sulfoalkyl starch, cyanoethyl starch, allyl starch, benzyl starch, carbamylethyl starch and dialkylamino starch; cross-linked starch such as methylol cross-linked starch, hydroxyalkyl cross-linked starch, phosphoric acid cross-linked starch and dicarboxylic acid cross-linked starch; starch graft copolymers such as starch-polyacrylamide copolymer, starchpolyacrylic acid copolymer, starch-polyvinyl acetate copolymer, starch-polyacrylonitrile copolymer, cationic starch-polyacrylic acid ester copolymer, cationic starchpolyvinyl copolymer, starch-polystyrene-maleic acid copolymer and starch-polyethylene oxide copolymer.
However, either of these desensitizing gum is inferior to gum arabic in the ability to desensitize the non-image areas of the lithographic printing plate to which they are applied.
On the contrary, the desensitizing ability of gum arabic is so strong that the ink receptivity of the image areas of the lithographic printing plate is substantially lowered if the gum arabic i used as the desensitizing gum and, therefore, it has often been encountered that a great deal of unacceptable printed matters must be produced before printed matters having a satisfactory ink density during press operation are produced.
As the gumming solution which can eliminate such a disadvantage in some degree, there has been known an emulsified gumming solution comprised of an oil phase consisting of a petroleum fraction containing a lipophilic substance such as a lipophilic surfactant soluble in organic solvents and a water phase containing a water-soluble organic polymeric compound. In such an emulsified gumming solution, the gum arabic exhibits not only the desensitizing ability but the stabilization effect for such emulsion as a protective colloid.
On the other hand, various kinds of the aforementioned water-soluble organic polymers exhibit only a low protective colloidal effect and they simply provide a gumming solution which causes the separation of the aqueous phase from the oil phase within several days